1. Field of the Invention
This invention relates to an exposure apparatus for producing an Ultra Large Scale Integrated Circuit (ULSI), a liquid crystal display panel and so on. In particular, the invention relates to an alignment system to adjust positions of masks and targets to be exposed.
2. Description of the Related Art
Light exposure apparatus have been applied to produce large scale integrated circuits. However, as the patterns of the circuits get finer and finer, it becomes harder to expose such circuit patterns by the light exposure apparatus because of resolution and productivity requirements.
Recently X-ray exposure apparatus have been developed instead of light exposure apparatus because the X-ray exposure apparatus can transfer finer circuit patterns than can light exposure apparatus.
In the case of such exposure by the X-ray apparatus, it is very difficult to transfer reduced patterns from a mask to a target such as a semiconductor wafer. Therefore, the patterns have to be transferred isometrically in an exposure process. In the case of isometric exposure, the gap between the mask and the wafer should have a minimum space of 10-50 .mu.m. Such exposure is called a proximity exposure.
A general X-ray exposure apparatus used to expose a semiconductor wafer is shown in FIG. 1. FIG. 1 is a schematic vertical sectional view.
In FIG. 1 X-ray 51 from syncrotron orbital radiation (SOR) is reflected on an X-ray reflect mirror (not shown) and passes through a port 52 kept in high vacuum pressure. After that, the X-ray passes through a window 53 and a bellow 54, then into a chamber 55. In the chamber 55 there re alignment systems 58 which detect the positions and distance between the X-ray mask 56 and the semiconductor wafer 60 by LASER beam 61 as a detecting beam. Behind the chamber 55 there is a mask stage 57 which holds and moves a X-ray mask 56 and a wafer stage 59 which holds and moves a semiconductor wafer 60.
In this structure, the X-ray 51 passing through the window 53 into the chamber 55 is exposed on the X-ray mask 56 and transfers patterns on the X-ray mask to the semiconductor wafer 60 which is spaced apart from the X-ray mask by dozens of .mu.m.
The exposure process as described above is repeated several times. Therefore, the positions and the distance between the X-ray mask 56 and the semiconductor wafer 60 must be detected in each exposure process and be adjusted to the right position precisely. The alignment system 58 is applied to the detection described above.
A conventional structure of such alignment systems are shown in FIG. 2 and FIG. 3.
Alignment marks are written on both an X-ray mask and a semiconductor wafer. The alignment system takes the picture data of these alignment marks and gets the information about the relative distance between the mask and the wafer by the processing of the picture data.
An explanation of the alignment marks written on the X-ray mask is described hereunder.
Three alignment marks (X mark 6a, Y mark 6b, .theta. mark 6c shown in FIG. 2 and FIG. 3) are written on an exposed area in the X-ray mask 6. Three optical alignment systems are installed in the alignment systems, the optical alignment system 11a for detecting the X mark position, the optical alignment system 11b for detecting the Y mark position and the optical alignment system 11c for detecting the .theta. mark position.
As shown in FIG. 2 and FIG. 3, these optical alignment systems 11a, 11b, 11c, comprise objective lenses (12a, 12b, 12c), three prisms (13a, 13b, 13c), and half mirrors and reflective mirrors (not shown in FIG. 2 and FIG. 3).
A detection beam passes through the half mirror and the objective lens and goes to the prism. The path of the beam is bent 90 degrees by the prism. After reflection by the prism the beam reaches the alignment mark. Then, the beam is reflected by the alignment mark and passes back the same way, i.e., through the prism and the objective lens. The beam passed through the objective lens reflects on the half mirror and the reflective mirror. The beam reflected on &he reflective mirror is processed as picture data expressing the position of the alignment mark.
The positions of the alignment marks 6a, 6b, 6c may change in each exposure processes and also may change depending on the chip sizes. Therefore, the optical alignment systems need to be moved according to the changes of the positions of the alignment marks. The optical alignment systems 11a, 11b and 11c are arranged on moving bases 16a, 16b and 16c respectively. The bases can move in X-Y directions individually.
The X-ray exposure apparatus is used in the proximity exposure. All of the alignment marks 6a, 6b, 6c for detecting should be in the exposure area, which is generally sized 10 mm to 30 mm in the shape of a square. The optical alignment systems 11a, 11b, 11c should be arranged over the alignment marks 6a, 6b, 6c, respectively, and they should not interfere with each other in such a narrow space.
Therefore, the moving bases 16b and 16c, opposite each other, are arranged on the same plane. However, the moving base 16a is arranged above the moving base 16b and 16c as described in FIG. 2 and FIG. 3. Thus the position of the optical alignment system 11a installed on the moving base 16a is above the other optical alignment systems 11b, 11c.
As shown in FIG. 2 and FIG. 3, the distance h.sub.2 from the optical axis of the optical alignment system 11b to the alignment mark 6b is equal to the distance h.sub.3 from the optical axis of the optical alignment system 11c to the alignment mark 6c (h.sub.2 =h.sub.3). However, the distance h.sub.1 from the optical axis of the optical alignment system 11a to the alignment mark 6a is bigger than the distance h.sub.2 or the distance h.sub.3 respectively (h.sub.1 &gt;h.sub.2, h.sub.1 &gt;h.sub.3).
When the optical alignment system 11a is not arranged in the same plane in which the optical alignment system 11b and 11c are arranged as described above, the focal length f.sub.1 of the object lens 12a attached in the optical alignment system 11a is different from the focal lengths f.sub.2 and f.sub.3 of the object lenses 12b and 12c attached in the optical alignment system 11b and 11c respectively.
The focal length f.sub.2 is equal to the sum of the length h.sub.2 and the length h.sub.2 ' which is the distance from the center of the lens 12b to the alignment mark 6b. The case of the focal length f.sub.3 is as same as f.sub.2. On the other hand, the focal length f.sub.1 is equal to the sum of the length h.sub.1 and the length h1' which is the distance from the center of the lens 12a to the alignment mark 6a. The value of f.sub.1 is not equal to the value of f.sub.2 or f.sub.3 respectively.
Therefore, the optical power of lenses are different from each other in these optical alignment systems and many different parts for producing the systems are needed. Also the production cost of these different systems is very high and production takes many processes. The width of X mark 6a, Y mark 6b and .theta. mark 6c are shown in FIG. 2 as c1, c2, and c3, respectively. The value of c1 is not equal to the value of c2 or the value of c3. The value c2 is equal to the value of c3 (c1.noteq.c2=c3). Because the optical power of the lenses are different from others it is required to detect the marks as a same scaled picture with a data processing system.